Light emitting diodes (LEDs) are attractive candidates for replacing conventional light sources such as incandescent lamps and fluorescent light sources. The LEDs have higher light conversion efficiencies and longer lifetimes. Unfortunately, an LED produces light in a relatively narrow spectral band. Hence, to produce a light source having an arbitrary color, a compound light source having multiple LEDs is typically utilized or part of the light from a single LED must be converted to light of a second wavelength, which is mixed with light from the original LED. For example, an LED-based white light source that provides an emission that is perceived as white by a human observer can be constructed by combining light from arrays of red, blue, and green emitting LEDs that are generating the correct intensity of light at each color. Similarly, light of other spectral emissions can be produced from the same arrays by varying the intensity of the red, blue, and green LED outputs to produce the desired color output. The intensity of light from each array can be varied by varying the magnitude of the current through the LED or by switching the LEDs on and off with a duty cycle that determines the average intensity of light generated by the LEDs.
A light source designer typically knows the desired output color for a light source in terms of standardized red, blue, and green light intensities. If the individual LEDs were highly reproducible and had light outputs that did not vary over the life of the LED, a light source constructed from red, blue, and green LEDs could be utilized to provide the intensities of the light from the individual colors by providing a drive circuit that drives each LED with the appropriate current or duty factor.
Unfortunately, the LED fabrication process provides LEDs having emissions and efficiencies that vary significantly from one LED to another even for LEDs of a particular type. In addition, the light output of an LED changes over time as the LED ages. If the designer constructs an LED lighting system by assuming that the LEDs are all the same, the variations lead to color shifts in the perceived spectrum of the light. Such variations are often unacceptable.
One solution to this problem involves selecting the LEDs such that the selected LEDs have precisely the correct emission efficiency and spectrum. Here, the individual LEDs are tested after fabrication. The LEDs that have the same output wavelength and current-to-light conversion efficiency to within some predetermined variation are grouped together. In essence, the LEDs are sorted into sub-types that have known parameters that are more tightly controlled. The light source designer can then specify a light source in terms of the sub-groups to avoid the manufacturing variability problem described above. Unfortunately, this solution reduces the production yield and increases the cost.
In addition, the sorting process does not solve the aging problems discussed above, and hence, the light source may still exhibit a color shift as it ages. To correct for the aging problem, a sensor is normally used to measure the intensity of light from the light source at the red, green, and blue wavelengths. A feedback control system then adjusts the drive currents or duty factors to the individual LEDs to maintain the color output at the desired point.
While a feedback system can correct for the aging of the LEDs, such systems are difficult to implement in a stand-alone light source designed to replace a conventional incandescent bulb. The LEDs are typically mounted on a first substrate in close proximity to one another. The light from the individual LEDs is mixed by a lens or other optical arrangement to provide the output light. The sensor must be located in the output light, and hence, must be located some distance from the LEDs. The controller drive circuitry is also typically mounted on a second substrate.